Resonant molecular transitions in second harmonic generation spectroscopy of Fe-octaethylporphyrin adsorbed on Cu(001)

Metal-organic molecular adsorbates on metallic surfaces offer the potential to both generate materials for future (spin-)electronics applications as well as a better fundamental understanding of molecule-substrate interaction, provided that the electronic properties of such interfaces can be analyzed and/or manipulated in a targeted manner. To investigate electronic interactions at such interfaces, we measure optical second harmonic generation (SHG) from iron-octaethylporphyrin (FeOEP) adsorbed on Cu(001), and perform electronic structure calculations using coupled cluster methods including optical excitations. We find that the SHG response of FeOEP/Cu(001) is modified at 2.15-2.35 eV fundamental photon energy compared to the bare Cu(001) surface. Our polarization-dependent analysis shows that the $\chi_{zzz}^{(2)}$ non-linear susceptibility tensor element dominates this modification. The first-principles calculations confirm this effect and conclude a resonantly enhanced SHG by molecular transitions at $\hbar\omega \geq 2$ eV. We show that the enhancement of $\chi^{(2)}_{zzz}$ results from a strong charge-transfer character of the molecule-substrate interaction. Our findings demonstrate the suitability of surface SHG for the characterization of such interfaces and the potential to employ it for time-resolved SHG experiments on optically induced electronic dynamics.